An aircraft such as a helicopter will normally have a main fuel tank and one or more inboard auxiliary tanks. It may also carry several outboard auxiliary tanks. For safety reasons it is desirable to be able to fuel the aircraft filling all tanks with no electrical power applied. Fuel is supplied under pressure to an inlet point for distribution to the various tanks.
Automatic hover in flight refueling is also desirable which permits controlled center-of-gravity refueling of the helicopter in flight.
As fuel in the main tank is used there comes a point where it is necessary to transfer fuel to the main tank from the auxiliary tanks. It is much preferred that this be automatic, either in response to a low level in the tank or in response to a pilot initiated actuating signal. Conveniently, flow transfer should operate without pilot intervention, but it is important that the pilot be alerted to any failure of the system.
Reliability and light weight of the system is important with penalties in the order of $250 a pound being charged for weight. Prior art refueling schemes required multiple flapper type valves and flow rate sensors which involved moving parts subject to wear and significant weight.
Fuel level, flow rate and pressure measurement traditionally relied on mechanical devices using moving parts which wear, break down, occupy large spaces, add substantial weight and incur maintenance costs with low reliability and accuracy.